State of the art cancer radiation therapy is increasingly based on the pinpoint application of high-energy radiation, which is highly tailored to the shape and position of the cancerous tumor. Modern techniques such as IMRT use a pencil sized treatment beam whose cross-section is shaped to match the tumor. This allows the physician to spare the surrounding healthy tissue while increasing the treatment dose to the cancerous target. As the size of the treatment beam decreases, the accurate location of the beam becomes much more critical. If a highly tailored beam is off target by a few millimeters, it may miss the tumor entirely.
Because of these new techniques, it becomes increasingly desirable to know the position and shape of the tumor accurately with the patient in the exact position that he will be at the time of treatment. For this reason, manufacturers of radiation therapy machines are increasingly combining their machines with built in diagnostic imaging capability. Advances such as On Board Imaging (OBI) and Cone Beam CT allow the development of treatment plans using tumor diagnostic imaging data while the patient is in the exact position and constraining configuration as during treatment. Image Guided Radiation Therapy (IGRT) techniques in which diagnostic imaging as well as treatment beams are used require that the couch top perform well in both modes.
Traditionally, patient treatment plans have been performed on a separate simulation machine, which uses diagnostic imaging either through static images or CT imaging. The patient is placed on a radiolucent tabletop also referred to as a couch top in the language of the industry. The patient couch top for diagnostic imaging application are optimized to provide minimum X-ray absorption in the 50 to 150 kVp range and are generally of a monocoque foam core/carbon fiber skin construction. Radiation Therapy patient tables generally operate in the 6 MeV to 25 MeV x-ray energy range (for photon energy treatment). At this energy level, an effect known as Compton Scattering occurs. As the photons pass through the couch top material, electrons are given off which then impact the patient. Electrons do not penetrate deeply into the human body but are rather absorbed by the skin and can cause skin reactions. For this reason, great care is taken in the design of Radiation Therapy patient couch tops to minimize the Compton Scattering effect. Consequently, couch tops developed for Radiation Therapy are generally of a different configuration than those made for diagnostic imaging.